What Do Bigfin Squid Eat and How Do They Catch It?

Nobody has ever watched a Bigfin squid eat. Despite a growing number of deep-sea sightings since the first confirmed footage in 1988, the diet and feeding behavior of Magnapinna remain among the most stubborn mysteries in marine biology. These squid live at crushing depths, typically around 3,000 meters on average, and their encounters with cameras last only minutes at best. What researchers have are educated guesses built from anatomy, a handful of observed postures, and comparisons with distantly related deep-sea squid. Those guesses point toward a slow, passive style of prey capture that looks nothing like the explosive ambush tactics of squid you might picture from nature documentaries.

Why the Diet Is Still Unknown

Bigfin squid are known almost entirely from fleeting ROV and submersible encounters and a small number of damaged net-caught specimens. No one has successfully kept one alive in a lab, and no stomach-content analysis has ever been published for an intact adult. The specimens that have been pulled up in trawl nets tend to arrive in poor condition, with their most distinctive feature, the extraordinarily long arm filaments, broken or missing entirely. Juveniles observed in the Great Australian Bight confirmed that even small individuals possess these long distal filaments, but the filaments are so fragile that they almost never survive collection.¹ Without intact specimens to dissect, the basic question of what goes in the animal’s mouth remains unanswered.

This is not unusual for deep-sea cephalopods. Many species living below 1,000 meters are known from fewer than a dozen specimens worldwide. But the Bigfin squid’s anatomy is so unlike anything else that even guessing at diet requires some creative reasoning.

The Filaments and What They Might Be For

The most striking thing about a Bigfin squid is the arms. All ten appendages, the eight arms and two tentacles, extend outward and then trail downward in thin, thread-like filaments that can stretch the animal’s total length to several meters. In some sightings, the filaments dangle far below the body like fishing lines. The “elbow” pose, where each arm bends at a roughly 90-degree angle before the filament drops straight down, is the posture most commonly captured on camera.

These filaments are almost certainly involved in feeding, though the mechanism is debated. The leading hypothesis is that Bigfin squid use them as passive contact traps. Small organisms drifting in the water column or crawling along the seafloor brush against a filament, stick to it or get tangled in it, and the squid then retrieves the filament to bring the food to its mouth. Think of it less like fishing with a lure and more like a spider’s web suspended in open water. The filaments may be coated in adhesive mucus or lined with microscopic suckers that grip anything that touches them, though confirming this from video footage alone is impossible.

Observations from the Great Australian Bight documented a behavior never previously seen in any squid: coiling of the distal filaments.¹ Coiling could serve several purposes. It might be a way of retracting and inspecting a filament after something has made contact. It could be a response to disturbance from the ROV itself. Or it could be part of a feeding sequence we simply haven’t seen play out to completion. Without being able to observe the animal undisturbed over long periods, interpreting any single behavior is speculative.

Clues from Body Orientation

One of the most informative observations came not from Magnapinna directly but from a closely related deep-sea squid with very large fins and extremely long arms, observed near the bottom at roughly 2,200 meters by an ROV. This animal oriented itself vertically with its head pointed downward and its tentacles extended toward the sediment, behaving in a manner consistent with bottom-contact feeding.² The same study described Magnapinnidae-like squid behaving similarly to mastigoteuthid squid, a family known for dangling tentacles toward the seafloor to intercept small benthic organisms.

This vertical, head-down posture is a recurring theme in deep-sea squid families that seem to feed passively. Rather than chasing prey through the water column, these animals position themselves above the bottom and let their appendages do the work. If Bigfin squid feed this way, their likely prey would include small crustaceans, polychaete worms, and other invertebrates living on or just above the deep-sea floor. Marine snow, the slow rain of organic particles from the surface, could also be part of the picture, though it would be an unusual food source for a cephalopod.

Not every sighting shows the head-down posture, though. The more commonly photographed “elbow” pose is sometimes horizontal rather than strictly vertical. Researchers in the Great Australian Bight documented a horizontal version of the elbow posture, suggesting the squid may adopt different orientations depending on whether it is targeting the seafloor, mid-water prey, or simply drifting.¹ A horizontal posture with filaments spread wide would maximize the cross-sectional area the animal covers, which makes sense if the goal is to intercept whatever drifts into contact.

A Body Built for Waiting, Not Chasing

Everything about the Bigfin squid’s physiology points away from active predation. Deep-sea cephalopods as a group have dramatically lower metabolic rates than their shallow-water relatives. Studies measuring metabolic enzyme activity in mantle muscle have found that deep-living squid can have enzyme levels hundreds of times lower than those of squid near the surface.³ The pattern holds across species and is thought to reflect a reduced need for powerful swimming in an environment where light is absent and visual predator-prey interactions barely exist.

Bigfin squid, living at a mean depth of roughly 3,000 meters, sit squarely in the zone where metabolic rates are expected to be at their lowest.⁴ Their large, paddle-like fins allow gentle, slow propulsion, not the jet-powered bursts typical of shallow-water squid. The gelatinous body tissue, another hallmark of deep-sea cephalopods, is lightweight and energy-cheap to maintain but provides very little muscle power. An animal this metabolically sluggish does not need much food, and it almost certainly does not expend significant energy obtaining what it does eat. The passive-trap feeding strategy fits this energy budget perfectly: deploy filaments, wait, retrieve whatever sticks.

This metabolic logic also helps explain why a diet of tiny, low-calorie organisms could sustain an animal that looks quite large on camera. The body itself, excluding those trailing filaments, is modest in size, and the energy demands of hovering in the deep ocean are minimal. A few small crustaceans or worms per day could conceivably be enough.

What Probably Isn’t on the Menu

Given the anatomy and behavior described above, some food sources can be tentatively ruled out. Bigfin squid are almost certainly not catching fish. They lack the muscular tentacles, sharp beaks, and burst-swimming capability that cephalopods use to subdue fish-sized prey. They are also unlikely to be scavengers feeding on large carcasses that fall to the seafloor, since their filaments seem designed for intercepting tiny organisms rather than tearing apart carrion.

It is also worth noting what Bigfin squid are not: they are not closely related to the giant squid or the colossal squid, both of which are active predators with powerful tentacular clubs armed with suckers or hooks. The Bigfin squid’s evolutionary lineage appears to have diverged early from other squid families, and their body plan represents a completely different solution to the problem of finding food in the deep ocean. Where giant squid evolved to be muscular ambush predators, Bigfin squid evolved to be patient, low-energy interceptors.

The Depth Range Complicates Things

One detail that makes dietary speculation harder is the surprisingly wide depth range where Bigfin squid have been recorded. While the average sighting depth is around 3,000 meters, the full range spans from about 1,578 meters down to over 6,200 meters.⁴ And intriguingly, nearly half of the specimen records show minimum depths in the upper 600 meters, suggesting that at least some individuals, possibly juveniles, spend time at much shallower depths than the adults typically seen by deep-diving ROVs.⁴

This matters for diet because the food web at 600 meters looks very different from the one at 4,000 meters. Shallower waters have higher densities of zooplankton, small fish, and other potential prey. If juvenile Bigfin squid live in the mesopelagic zone before descending to the abyss as they mature, their diet could shift substantially over their lifetime. Many deep-sea cephalopods undergo ontogenetic descent, living in relatively shallow water as juveniles and migrating to deeper habitats as adults, and a dietary shift would be expected to accompany that transition. Whether Bigfin squid actually do this, and what they eat at each stage, remains an open question.

How Other Deep-Sea Squid Feed

The best analogues for Bigfin squid feeding come from mastigoteuthid squid, the family whose behavior Magnapinna most closely resembles in ROV footage. Mastigoteuthids are known to orient vertically with long tentacles dangling toward the seafloor, and gut-content analyses of some mastigoteuthid species have revealed diets consisting mainly of small crustaceans and other zooplankton. The behavioral resemblance between Magnapinna and mastigoteuthids noted by researchers observing both families from submersibles is one of the strongest pieces of indirect evidence for what Bigfin squid might eat.²

Another useful comparison is the vampire squid, Vampyroteuthis infernalis, which lives at similar depths and was long assumed to be a predator before researchers discovered it actually feeds on marine snow and detrital aggregates. The vampire squid uses sticky filaments to collect falling organic particles, a strategy that would be easy to imagine for Bigfin squid as well, though the two are not closely related and the comparison is purely functional. The deep ocean has independently produced the same basic feeding solution, dangling sticky appendages in the path of whatever drifts by, in multiple unrelated lineages, which suggests the strategy works well in environments where food is scarce and chasing it is costly.

Why Direct Evidence May Take Decades

Getting a definitive answer to what Bigfin squid eat will require either a successful stomach-content analysis of an intact specimen or long-duration video of undisturbed feeding behavior. Both are extraordinarily difficult with current technology. ROV encounters are typically brief and disruptive; the lights and thrusters of a submersible alter the animal’s behavior the moment it is detected. Environmental DNA sampling from water near sighting locations might eventually reveal prey species in the vicinity, but linking that to actual consumption would be a stretch.

The most promising approach may be improved deep-sea camera traps, autonomous systems that can sit on the seafloor for weeks or months recording whatever passes by without the disturbance of an active vehicle. Several oceanographic institutions are developing these systems for general deep-sea monitoring, and the accidental capture of Bigfin squid feeding on such a system would be a landmark observation. Until then, the animal’s diet lives in the gap between what its body can tell us and what its behavior hints at: almost certainly small, slow-moving organisms captured passively by those remarkable dangling filaments, brought to the mouth one tiny morsel at a time in the silent darkness of the abyss.

Magnapinna Sightings and the Geography of Observation

Bigfin squid have now been recorded in every major ocean basin, from the Gulf of Mexico to the Indian Ocean to waters around Australia and New Zealand. The global distribution itself is not surprising for a deep-sea animal that occupies the vast bathypelagic and abyssal zones, but it does mean that wherever researchers eventually observe feeding, the behavior is likely to be representative of the genus as a whole rather than a quirk of one local population. The Great Australian Bight observations were particularly valuable because multiple individuals were recorded across different dives, allowing researchers to compare postures and behaviors across sightings rather than relying on a single encounter.¹ Each new sighting adds to a slowly growing behavioral catalog that, piece by piece, is narrowing the range of plausible feeding strategies even without a single confirmed meal.